Dynamic stabilizer for plasma instabilities to improve plasma confinement and to increase plasma density

ABSTRACT

Method and apparatus for dynamic stabilization of plasma instabilities for improving plasma confinement and for increasing plasma density independent of the phase of the instabilities. The method comprises confining the plasma column between ionizing plates which supply energetic particles to the plasma in a direction parallel to the axis of the plasma column and interacting the plasma in a predetermined direction with externally supplied radio-frequency fields having a selectively variable power level amplitude and a sufficiently high frequency range.

United States Patent I Hendel et al.

[ 51 June 6,1972

[54] DYNAMIC STABILIZER FOR PLASMA INSTABILITIES TO IMPROVE PLASMA CONFINEMENT AND TO INCREASE PLASMA DENSITY [72] lnventorsr Hans W. llendel, W. Princeton; Chu, Tsu- Kai; Thomas C. Simonen, both of Princeton, all of NJ.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission 221 Filed: Feb. 18,1970

2 11 Appl.No.: 12,310

3,031,399 4/1962 Warnecke et al 176/2 3,052,614 9/1962 Herold ..176/2 3,090,737 5/1963 176/2 3,418,206 12/1968 Hall et al. 176/5 OTHER PUBLICATIONS Plasma Physics and Controlled Thermonuclear Fusion edited by K. D. Sinelnikov Translated from Russian, Pub. by NASA Library N0.QC718 K6 pp. l5, l6, 17, 18

Primary Examiner-Reuben Epstein Attorney-Roland A. Anderson ABSTRACT Method and apparatus for dynamic stabilization of plasma instabilities for improving plasma confinement and for increasing plasma density independent of the phase of the instabilities. The method comprises confining the plasma column between ionizing plates which supply energetic particles to the plasma in a direction parallel to the axis of the plasma column and interacting the plasma in a predetermined direction with externally supplied radio-frequency fields having a selectively variable power level amplitude and a sufficiently high frequency range.

4 Clains, 2 Drawing figures '\.J 42 45 \J CONTROL ACCESS PORTS ruuesrsu 25 COILS 1 I l END PLATE VACUUM ELECTRON TANK BOMBARDMENT FILAMENT PUMP OVEN

' Plasma column axis PAiENTEllJull 5 I972 3. 668 O66 SHEET 2 [IF 2 Axial magnetic 25 i confining means 83 (Source) Axial field lines Stabilization Destabilization .Swpe detector Slope detector amp. backs off amplitude DENSITY RF STABILIZING AMPLITUDE Fig. 2

INVENTOR.

HANS W. HENDEL BY TSU-KAI CHU THOMAS C. SIMONEN KW WW DYNAMIC STABILIZER FOR PLASMA INSTABILITIFS TO IMPROVE PLASMA CONFINEMENT AND TO INCREASE PLASMA DENSITY BACKGROUND OF THE INVENTION In the field of plasma physics, it is desirable to confine plasma in a magnetic field. Various proposals have been made and used for such plasma confinement, comprising the arrangements described and shown in Project Sherwood The U. S. Program in Controlled Fusion by Amasa Bishop, Addison Wesley Publishing Co., 1958. However, as described in the co-pending application, Ser. No. l2,309, filed Feb. I8, 1970, entitled Plasma Control by Feedback," by Hans W. Hendel et al, the co-inventors of this application, which is assigned to the assignee of the former application and filed concurrently therewith, these heretofore known systems have had low plasma confinement times, density, and/or temperature,

at least partially due to instabilities. Stabilization by static magnetic geometries has been complicated, difficult or expensive to build and operate. Thus, it is desirable to provide a dynamic plasma instability suppression means which, for simplicity, operates independently'of the phase ofthe instability.

SUMMARY OF THE INVENTION This invention, which was made in the course of, or under a contract with the United States Atomic Energy Commission,

provides a simple and effective system for suppressing plasma instabilities by applying rf'fields to the plasma at a frequency above the ion cyclotron frequency. In this regard, the method and construction involved in this invention utilizes standard and well-known techniques for forming plasmas and confining them in a magnetic field. As such, this invention is highly flexible for the wide range of plasma forming means, applications, linear and'toroidal magnetic confinement geometries, plasma constituents, and plasma energies, temperatures, densities and confinement times to which the previously employed plasma research reactors have been employed. More particularly, in one embodiment, this invention is applied to the conventional magnetically confined plasma column in the Q machine at Princeton University for increasing the plasma confinement and density therein. To this end, in accordance with the discovery of this invention, radio-frequency energy, particles or momentumare injected into the plasma independently of the phase of the instability at a frequency close to or above the ion cyclotron frequency of the plasma. With the proper selection of elements and parameters as described in more detail hereinafter, the desired plasma instability suppression is achieved and the plasma confinement and density are correspondingly increased. 7

The above and further novel features and objects of this invention, will appear more fully from the following detailed ,description of one embodiment thereto when read in connection with the accompanying drawings, and the novel features will be particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAMNGS FIG. 1 is a partial cross section of a conventional Q machine, which employs the plasma instability suppression means of this invention. FIG. 2 is a partial schematic view of V another embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT opposite ends of a longitudinally extending cylindrical vacuum container 19 for receiving injected plasma particles from the plates 15 and 17 and confining them in a column 21 along an axis 23 in a uniform axial confining field provided by a conventional solenoid 25, the axis of which coincides with the axis 23 of plasma column 21 and the Z axis of container 19. In the Q-l machine at Princeton University, the plasma column 21 is confined away from the inside wall 27 of the container 19 with densities up to 3 X 10" plasma particles/cm, operating at a temperature of 2,700 K for extended periods of time. To this end, a uniform axial field of about 6,000 gauss is produced by solenoid 25 to form field lines 29 directed along and parallel with the axis 23 of plasma column2l, whereby the field lines 29 confine the plasma in concentric magnetic surfaces, which are well known in the art and one of which is illustrated herein by a magnetic surface forming amagnetic aperture identified by reference number 31, whereby the plasma is confined away from the inside wall 27 of container 19. Advantageously, the plasma 33 in column 21 is formed from potassium or cesium that is ionized and injected by and from plates 15 and 17, and neutralized by the electrons emitted therefrom. In this regard, filaments 35 and 37 heat the plates 15 and 17 to high temperatures, corresponding to the temperature of the plasma 33.

In accordance with experiments by the co-inventors herein, a collisional drift instability, which is referred to hereinafier as the Universal Instability, is produced by increasing the described axial field strength above a critical value. This Universal Instability has been described in detail by these coinventors in Phys. Rev. Letters 18, 439 (1967) by a comparison with the linear theory of the dependence of its oscillation frequency w and its wave vector k. In this regard, as the axial magnetic confining field produced by solenoid 25 increases, successive azimuthal modes destabilize when the electron-fluid expansion rate along the field lines 29 exceeds the diffusion rate of ions across the field due to ion-fluid viscosity. Concommitant with instability onset, wave induced plasma loss occurs from the magnetically confined plasma column 21 and the plasma density therein decreases.

This invention hereinafter described utilizes a plasma column 21 to which radio-frequency fields are applied at a frequency close to or above the ion-cyclotron frequency of the plasma 33 and independently of the phase of the Universal lnstability occurring in plasma 21, thereby to suppress the instability.

In this regard, a conventional radio-frequency signal source 41, having suitable means 42 providing controls 43 and 45 for respectively, continuously varying the frequency and amplitude of the signal, supplies rf energy to the hot plates 15 and 17. In actual practice, for example, the rf fields have been applied to plates 15 and 17 at a frequency of 10 mega hertz,which frequency is well within the range of conventional rf energy sources. In one example, this application of the rf energy to plates 15 and 17 has suppressed the Universal Instability amplitude and has correspondingly increased the plasma density in plasma column 21. In this regard, the frequency and amplitude of said rf signal from signal source 41 are advantageously continuously varied by control 42 for maximizing the density of the plasma 33 in column 21 in accordance with the increasing density and confinement of the plasma 33 in column 21 as said instabilities are suppressed. In one example, suitable density detector 47,comprises a conventional plasma density responsive light interferometer. One such interferometer is shown and described in the above cited copending application by the co-inventors of this application.

While the above has described this invention as it is applicable to providing a simple and effective system for dynamically suppressing the Universal Instability in the plasma column confined in a uniform magnetic field in the 0-1 machine at Princeton University, it will be understood from the above that the method and apparatus of this invention are applicable to suppressing the Universal Instability or other instabilities in any plasma confined in magnetic fields provided in any of the wide variety of plasma research devices.

Another embodiment is shown in FIG. 2. The rf field is applied across the plasma or to a part of it. The density of the plasma is monitored and fed into a density slope detector 74 which feeds into a processor 75 to apply more power as long as the density increases (positive slope) and which reduces the power when the density decreases (negative slope). Thus, the density will be optimized by optimum dynamic stabilization. The rf source 83 can be replaced by other electromagnetic sources.

This invention has the advantage of providing dynamic stabilization of instabilities in a magnetically confined plasma. Moreover, the system of this invention is simple, effective and easy to operate independently of the phase of instabilities in the plasma, In this regard, actual tests have shown that this invention can be combined with standard plasma confinement means, such as Q machines, actually to increase the density and confinement of the plasma over the density and confinement times obtained without its application.

We claim:

1. Apparatus for controlling plasma instabilities that produce plasma losses from a magnetically confined plasma column, comprising cylindrical vacuum container means having a longitudinally extending axis and an inside wall terminating in opposite ends for containing said plasma, means having a coil around said container means for providing a uniform axial magnetic field for confining said plasma in a column that is spaced from the inside wall of said container means, said magnetic field having a strength capable of producing plasma wave instabilities in said plasma column and field lines directed parallel to said axis of said container means for forming concentric magnetic surfaces for confining said plasma column along said axis between said opposite ends of said container means with predetermined ion-cyclotron frequencies, particle ionizing plates at said opposite ends of said container means having filaments for heating said plates to high temperatures corresponding to the temperature of said plasma in said plasma column and an oven for directing atomic elements against said plates whereby plasma particles in the form of ions and electrons are injected into said container means along the axis thereof to form and to maintain said plasma in said column at a predetermined temperature for long periods of time, said plasma ions being neutralized by the electrons injected into said plasma column from said plates, and means connected to said plates for transmitting into said plasma energy in the form of electromagnetic waves having selectively controlled frequencies and amplitudes, said waves being directed into said plasma column from said plates parallel to said axis of said container means for controlling said instabilities in said plasma column for controlling the plasma losses from said plasma column to said inside wall of said container means that correspond to the amplitude of said instabilities.

2. The invention of claim 1 having first means for determining the density of said plasma in said column for selectively adjusting the frequency and amplitude of said waves in accordance with the density of said plasma.

3. The method of stabilizing a magnetically confined plasma column of high temperature ions andelectrons by stabilizing low. frequency instabilities therein so as to increase the density of said plasma, comprising the steps of confining said plasma column in a vacuum chamber between ionizing plates for supplying energetic particles to said plasma in a direction parallel to the axis of said plasma column, and interacting said plasma in a predetemiined direction with externally supplied radiofrequency fields having a selectively variable power level amplitude and a sufficiently high frequency range for effecting the amplitude of said instabilities in accordance with the amplitude and frequency of said externally supplied fields that are interacted with said plasma, whereby said instabilities are stabilized for maintaining a plasma having a stable dynamic equilibrium of at least 2 X 10 plasma particles /cm and said radio-frequency fields have a frequency of 10 megahertz.

4. Method of dynamically controlling the density of an ionized plasma confined in a magnetic field by forming particles in a plasma column at a first rate, measuring the density of said plasma for deternumng the plasma loss rate from said column, and maintaining a balance between said aforementioned rates by selectively transmitting radio-frequency fields into said plasma at a frequency above the ion cyclotron frequency of said plasma, said radio-frequency fields being continuously varied to increase the density of said plasma in accordance with the rate of said continuous variation for effecting the maximizing of said plasma density. 

2. The invention of claim 1 having first means for determining the density of said plasma in said column for selectively adjusting the frequency and amplitude of said waves in accordance with the density of said plasma.
 3. The method of stabilizing a magnetically confined plasma column of high temperature ions and electrons by stabilizing low frequency instabilities therein so as to increase the density of said plasma, comprising the steps of confining said plasma column in a vacuum chamber between ionizing plates for supplying energetic particles to said plasma in a direction parallel to the axis of said plasma column, and interacting said plasma in a predetermined direction with externally supplied radio-frequency fields having a selectively variable power level amplitude and a sufficiently high frequency range for effecting the amplitude of said instabilities in accordance with the amplitude and frequency of said externally supplied fields that are interacted with said plasma, whereby said instabilities are stabilized for maintaining a plasma having a stable dynamic equilibrium of at least 2 X 1012 plasma particles /cm3 , and said radio-frequency fields have a frequency of 10 megahertz.
 4. Method of dynamically controlling the density of an ionized plasma confined in a magnetic field by forming particles in a plasma column at a first rate, measuring the density of said plasma for determining the plasma loss rate from said column, and maintaining a balance between said aforementioned rates by selectively transmitting radio-frequency fields into said plasma at a frequency above the ion cyclotron frequency of said plasma, said radio-frequency fields being continuously varied to increase the density of said plasma in accordance with the rate of said continuous variation for effecting the maximizing of said plasma density. 